sedimentary
Chert (Chalcedony)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Tan, brown, or white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.6-2.65
- Hardness
- 6
- Color
- Tan, brown, or white
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Tan, brown, or white; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (has conchoidal fracture); Specific gravity: 2.6-2.65
Formation & geological history
Forms as nodules or layered deposits through the accumulation of silica-rich fluids in sedimentary environments, often involving siliceous microorganisms like diatoms or radiolarians. May be found in limestone or chalk beds.
Uses & applications
Historically used for stone tools and arrowheads due to sharp edges; currently used for road ballast, lapidary arts, and gem collecting.
Geological facts
Chert is so hard it can strike sparks from steel. Because it breaks with a conchoidal (shell-like) fracture, it was the primary material for the prehistoric 'Stone Age' tool industry.
Field identification & locations
Identify by its waxy appearance and smooth, curved fracture surfaces. It is translucent to opaque; can be found in pebble or cobble form in riverbeds or coastal cliffs.
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